Why do the pilgrims wear those clothes?

Praise be to Allah.

Allaah commanded us, on the lips of His Messenger Muhammad SAWS (peace and blessings of Allaah be upon him) to wear the izaar (lower garment) and rida’ (upper garment) during Hajj and ‘Umrah for a reason that is known to Him. So we must obey in the hope of earning reward, whether we know the reason or not. What the scholars have said concerning that is that it is a reminder to the people of the Day when they will be raised and gathered together, the Day of Resurrection, and to make the pilgrim feel humble and that rich and poor are equal. We ask Allaah to give us and you strength and to help us remain steadfast in adhering to the truth until we meet Him.

And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad.